Hollow fiber membrane and braided tubular support therefor

ABSTRACT

An asymmetric membrane comprising a tubular polymer film in combination with a tubular braid on which the film is supported, requires the braid be macroporous and flexible, yet sufficiently strong to withstand continuous flexing, stretching and abrasion during use for microfiltration (MF) or ultrafiltration (UF). The specifications for a braid of a long-lived membrane are provided. A membrane is formed by supporting a polymer film in which particles of calcined α-alumina are dispersed, on the defined tubular braid.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. Ser. No. 10/037,432, filedJan. 4, 2002, which is a continuation-in-part of U.S. Ser. No.09/335,073, filed Jun. 17, 1999, and a continuation-in-part of U.S. Ser.No. 08/886,652, filed Jul. 1, 1997, now U.S. Pat. No. 5,914,039.

FIELD OF THE INVENTION

This invention relates to a braided tubular support for a film ofpolymer which functions as an asymmetric semipermeable membrane inmicrofiltration (MF) and ultrafiltration (UF) applications. The braidedtube is no more than about 3 mm in outside diameter and relies on thepolymer film to imbue the fiber membrane product with sustainable highflux along with sufficient abrasion resistance such that a skein offibers (also referred to as a “module”) can operate in a commercialfiltration application for several months without the formation ofpin-holes.

BACKGROUND OF THE INVENTION

U.S. Pat. No. 5,472,607 to Mailvaganam Mahendran et al discloses ahollow fiber semipermeable membrane in which a tubular macroporoussupport is superficially coated on its outer surface with a thin film ofpolymer, most preferably of polyvinylidene difluoride. The tubular braidis flaccid but other details of the structure of the braid are notspecified. For example, the effect of characteristics of the materialforming the braid were not known; nor was the effect of a cross-sectionwhich was not truly circular, i.e. having “cylindricity” substantiallyless than 1.0. The term “cylindricity” (sometimes referred to as“roundness”) refers to how perfectly the circular cross-section of thetubular support matches the geometry of a true circle drawn tocorrespond to the mean diameter of the braid, a perfect match being 1.0.It was therefore not known at that time, how critical the physicalcharacteristics of a preferred braid were to the performance of a hollowfiber membrane using the braid.

In commercially available braid, made with conventional braidingequipment from commercially available yarn, there were numerous “breaks”in the fiber; also, accumulation of clumps of broken filaments, referredto as “fuzz”, braided into the cylindrical wall of the braid, resultedin weak spots in the polymer film coated onto the surface; and brokenfilaments, referred to as “whiskers”, protruding from the surface of thetubular braid, resulted in too-thick domains of polymer which wereconcentrated around the whiskers; and, when the domain was nottoo-thick, whiskers have a proclivity to initiate pin-holes.

Further, if the open weave of the braid provided either too high or toolow a braid porosity as measured by resistance to air flow, the fibermembrane formed was unusable in a commercial installation. Too open aweave resulted in the braid being embedded, that is, enclosed by andfirmly fixed in the polymer which also infiltrates into the bore of thebraid; thus, too open a weave results in greatly reduced permeability.Too tight a weave results in the polymer not being anchored sufficientlywell on the surface; this increases the likelihood that, in service, thepolymer film will be peeled from the braid. When operating flux wasexcellent, portions of the polymer film were sometimes found to havebeen peeled away when the fibers were backwashed with clean water orother fluid medium, whether water or permeate, under pressure; orportions of the film were “blown off” the surface of the fibers whentheir lumens were pulsed with air under pressure. Even with the bestbraid produced under controlled conditions, shrinkage during usage in anaqueous medium varied unpredictably. This resulted in taut fibers whichwere prematurely fouled because they were unable to move sufficiently tostay clean or rub against each other. If too taut, the fibers are brokenbefore they are fouled, or torn from potting resin in the header.Particularly because it is essential for best performance, and to shedcontaminants from the surfaces of the hollow fiber membranes, that askein of fibers operate with “slack” fibers, the structure of the braidneeds to survive repetitive twisting, and it was not known what physicalcharacteristic(s) of the braid was conducive to such survival. Acylindricity less than 0.8 resulted in a resinous filaments essentiallyinsoluble in the solvent in which the membrane-forming polymer isdissolved, the braid having a stable heat-pre-shrunk length which is inthe range from about 1% to 20% less than its unshrunk length, preferablyso that, irrespective of the material forming the fibers, when the preshrunk braid is stretched longitudinally, it has “give”, that is, theextension at break is at least 10%, preferably in the range from 10% to30%, and more preferably about 20%.

It is a specific object of this invention to provide a heat-pre-shrunktubular braid made with specified patterns, using carriers carrying yarnhaving defined number of filaments, ends, denier, and picks, underconditions which control the porosity (measured as permeability to air)of the braid, such controlled porosity serving to anchor a polymer filmnon-removably on the surface of the tubular braid.

It is another specific object of this invention to provide, in aflexible macroporous tubular braid support for an outside-in hollowfiber asymmetric membrane having a tubular film of synthetic resinousmaterial supported on the outer circumferential surface of the braidwithout the support being embedded in a thin film having a wallthickness of less than 0.2 mm, the improvement comprising, 16 to 60separate yarns, each on its own carrier, each yarn being multifilament150 to 500 denier (g/9000 meters) yarn, each multifilament being madewith from 25 to 750 filaments, each filament being from 0.5 to 7 denier.From 1 to 3 multifilament ends constitute a yarn, and the individualends are most preferably not plied together, but lie linearly adjacentto each other until taken up in the “fell” of the braid being woven. Thebraid being woven has from 30 to 45 picks (crosses/inch). The higher thedenier of the filaments, the fewer the filaments used, but the braidwall thickness is maintained in the range from about 0.2 mm but lessthan three times the diameter of the yarn from which the braid is woven,preferably less than 1.0 mm. The air permeability of the braid ofsynthetic resinous yarn is in the range from about 1 to 10 cc/sec/cm² ata differential pressure of 1.378 kPa (0.2 psi); and the moisture regainis in the range from about 0.2% to 7% by weight (wt). The finished fibermembrane is coated with a thin polymer film having a thickness in therange from 0.05 mm to 0.3 mm, most preferably less than 0.1 mm thick.The film has an annular peripheral barrier layer or “skin”circumferentially integral with successive microporous layers in thefilm, each layer contiguous with a preceding layer, the layers includingan outer annular layer, an intermediate transport layer, and an annularinner layer.

BRIEF DESCRIPTION OF THE DRAWING

The foregoing and additional objects and advantages of the inventionwill best be understood by reference to the following detaileddescription, accompanied with schematic illustrations of preferredembodiments of the invention, in which illustrations like referencenumerals refer to like elements, and in which:

FIG. 1A schematically illustrates a “diamond” pattern in a tubularbraid.

FIG. 1B schematically illustrates a “regular” pattern in a tubularbraid.

FIG. 1C schematically illustrates a “Hercules” pattern in atubularbraid.

FIG. 2 is a cross-sectional elevational view along a longitudinal axis,of a coating nozzle used to form the thin non-supporting film membraneon the braid.

FIG. 3 is a cross-sectional end view of a hollow fiber membrane of thisinvention schematically illustrating the radially disposed annular zoneswhich extend longitudinally axially over the length of the membrane, andshowing how the tubular non-self-supporting film is supported on thebraid without being embedded therein.

FIG. 4 is a cross-sectional view with greatly enlarged dimensions, toillustrate the dimensional relationships of pores in the componentlayers of the braid-supported membrane which pores make the membrane soeffective, particularly for microfiltration and ultrafiltration.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Details of a hollow fiber membrane are presented in the aforementioned'607 patent and Ser. No. 08/886,652 application, the disclosure of eachof which is incorporated by reference thereto as if fully set forthherein. A preferred tubular braid is woven with yarn, the denier ofwhich is chosen with consideration of the outside diameter of the braidon which the polymer film is to be coated, and whether the membrane isto be used for MF or UF. A desirable air-permeability for a UF membraneto provide drinking water, is in the range from about 5 to 25 LMH/kPa(liters/m²/kPa/hr) or 20 to 100 GFD/psi (gals/ft²/day/psi), preferablyfrom about 7.4 to 18.5 LMH/kPa (30 to 75 GFD/psi), measured with RO(reverse osmosis) water; a desirable permeability for a MF membrane usedto filter municipal sewage and provide clean water is in the range from10 to 50 LMH/kPa (40 to 200 GFD/psi), typically about 12.5 to 25 LMH/kPa(50 to 100 GFD/kPa), measured with RO water. A typical defect-free fiberhas a bubble point in the range from about 140 to 280 kPa (20 to 40psi). For a UF membrane it is desirable to have a bubble point in therange from 13 to 40 kPa (2 to 6 psi), preferably about 35 kPa (5 psi) toemphasize the importance of a defect in a fiber; for a MF membrane it isdesirable to have a bubble point in the range from 6 to 20 kPa (1 to 3psi), preferably about 13 kPa (2 psi), for the same reason.

The structure of the tubular braid is determined by the machine used toweave the braid which is formed of intertwined, spiral yarns so that itsthickness is less than three yarn diameters, and the yarn orientation ishelical. The braided tube may be woven on either vertical or horizontaltubular braiding machines, the former being preferred. A machineincludes a track plate provided with intertwining tracks, plural tube orbobbin carriers for the yarn capable of moving counterclockwise orclockwise along the tracks for braiding, a former and a take-up device.Bobbins are flanged tubes used for yarns which are difficult to handle.Yarns from bobbins mounted on the bobbin carriers are braided as theyare guided to a gathering guide disposed above the center of the disk.Each bobbin carrier is rotated by a drive gear disposed under the trackplate while it moves along the tracks. The ratio between the movingspeed of the bobbin carriers and the braid drawing speed can be changedby changing the gear ratio, so that the braids may differ from eachother in the angle of the strands. Different interlacings, or weavepatterns, can be achieved by controlling the motion of the yarncarriers. By controlling the take-up rate, the angle of the braid can becontrolled. It is essential that the yarn tension be controlled toprovide uniform tension so as to form a uniform braid. Machines formaking the tubular braid and the method of making it are well known andform no part of the invention. If desired, axial reinforcements may beprovided by using a third system of yarns which can be inserted betweenthe braiding yarns to produce a triaxial braid. Such reinforcement istypically found unnecessary.

A typical tubular braid is made from two sets of yarns or ends which areintertwined. Preferred materials are polyesters and nylons in yarn whichis most preferably in the range from about 200 to 400 denier (g/9000meters), with from 40 to 100 filaments having a denier in the range fromabout 3 to 6. The braid is preferably woven with from 16 to 28 carrierswith from about 36 to 44 picks (crosses/inch) to have an outsidediameter in the range from about 1.5 mm to 2.5 mm and a wall thicknessin the range from about 0.15 mm to about 0.50 mm, most preferably about0.3 mm.

The pattern in which the braid is woven is not narrowly criticalprovided the porosity is maintained within chosen limits, and though a“Regular” or “Hercules” braid is usable, a “Diamond” pattern is mostpreferred. Referring to FIG. 1A, there is schematically illustrated adiamond braid having an alternation of one yarn passing above and thenbelow the other yarns (1/1). FIG. 1B illustrates the regular braidpasses above two and below two in a repeat (2/2). FIG. 1C illustratesthe Hercules braid which has a structure of 3 up, 3 down (3/3).

The load at break of a preferred heat-shrunk braid is at least 50lb-force, preferably from 444 to 888 Newtons (100 to 200 lb-force),recognizing that the heat-pre-shrunk synthetic resinous braid has astable length which is in the range from 1% to 16% less than itsunshrunk length.

The critical importance of providing a stable heat pre-shrunk length isbecause the outer surfaces of taut fibers in a skein (taut because ofshrinkage during use) become so fouled that they are ineffective tofilter. Further, stresses on taut fiber membranes stress not only thetubular braid but the overlying polymer film. Undue stress on the braidresults in breakage, typically near the ends of the fiber membranes,where they are potted in headers; and undue stress on the polymer filmdiminishes its adherence and increases its susceptibility to peelingfrom, or sloughing off the surface of the braid. Though a “shrink test”is commonly conducted on yarns by heat shrinking in water at 98° C. viaa Texurmat boil off; or, in dry air at 177° C. with 0.045 gf/dtextension for 2 mm (DuPont); or, in dry air at 190° C. with 0.135 gf/d for30 sec (Monsanto), to date there has been no reason to heat pre-shrinkany tubular braid of synthetic resin, prior to its being coated withpolymer for the stated purpose of this invention, namely to makeoutside-in hollow fiber microfiltration and ultrafiltration asymmetricmembranes. More particularly, since a braid woven with glass fiber isessentially non-heat-shrinkable, there has been no reason to provide astable length of a polyester or nylon tubular braid by pre-shrinking itso that its shrunk length is about 84% of its pre-shrunk length at thesame time ensuring that the braid retains at least 95% of its tensilestrength.

Heat-shrinking in dry air, referred to as Testrite tests, of polyesterand polyamide tubular braids to obtain most preferably from about 16% to18% shrinkage, may be achieved in an electric furnace at 232° C. for 29sec.

The denier of the yarn and structural characteristics of the braiddetermine the liquid and gas permeability. The liquid permeability ofthe braid is at least one order of magnitude (that is, more than 10times) greater than the permeability of the polymer film. Thus the weaveof the braid is so open that it presents an insubstantial barrier to gasflow.

Permeability to air of preferred polyester (“PE”) and nylon (“NY”)braids, determined by ASTM Standard “Air Permeability of Textile FabricsD 737-96” are measured for a differential pressure of 1.38 kPa (0.2psi). These are listed in the following Table 1 under “@0.2 kPa”. Alsolisted are permeabilities “@0.02 kPa (0.029 psi) which are obtained byextrapolation of the data curve obtained with measurements at 0.2 kPa,in the appropriate range: TABLE 1 Permeability cc/sec/cm² I.D. O.D.Length Area Flowrate @1.38 Sample mm mm mm cm² cc/sec kPa @0.2 kPa PE-10.76 1.55 30 1.461 10.36 7.09 1.03 PE-2 1.02 1.89 28 1.663 7.05 4.240.62 PE-3 1.13 2.04 26 1.666 5.00 3.00 0.44 PE-4 0.76 1.89 25 1.484 1.521.02 0.17 NY-1 1.00 2.10 27 1.781 3.94 2.21 0.32 NY-2 0.89 1.86 27 1.5784.29 2.72 0.37 NY-3 1.28 2.04 23 1.474 3.93 2.67 0.37

The moisture regain values for polyester braids are in the range fromabout 0.2% to 0.5% by wt., and for the above PE samples, are from about0.4% to 0.5% by wt. For nylon braids moisture regain values are in therange from 4% to 7% by wt., and for the above NY samples are from about4% to 5% by wt. The structure of the tubular braid provides an outersurface which is uniquely configured to have a membrane's polymer filmadhere to the surface sufficiently so as not to be detached when themembrane is backwashed; the polymer film is held by the upper portion ofthe wall of the braid without having the wall embedded in the film. Thedegree of adherence is affected to some extent by the affinity of thechemical composition of the polymer for the material of the braid, butto a greater extent by the structure of the braid. The polymer film maybe any polymer which provides a satisfactory asymmetric membrane, andmay be formed from a polyester, polyamide, polyolefin, poly amine,polyurethane, polysulfone or cellulose acetate, most preferably PVDFcontaining calcined a-alumina as disclosed in U.S. Ser. No. 08/886,652,the disclosure of which is incorporated by reference thereto as if fullyset forth herein.

The test of establishing whether adherence is satisfactory is determinedby a Peel Test Procedure carried out on a Lloyd Instruments “MaterialsTester” (LR K5 with a 50 N load cell) having a “German Wheel” (the“Tester” for brevity).

The “German Wheel” is used to execute a peel test of a coating on aflexible substrate, at 90° to the surface of the substrate. Each sampleis especially prepared according to the standard being used. The GermanWheel consists of a free running axle mounted wheel and a yoke whichreceives the wheel and connects it to the load to execute a test. Theface of the wheel contains a sharp angled slot into which one end of thecoated substrate is inserted and folded back against the sharp edge.This creates a mechanical lock which holds the sample tight as itslength is drawn, coating side up, around the periphery of the wheel andpassed through a locking clamp. The clamp site is just beyond a regionwhere the coating tab length has been separated from the substrate. Thusthe flexible substrate is clamped and the coating tab length hangsfreely, in front of the clamp.

All tests are done on wet membranes, by slitting a six inch (6″) wetmembrane longitudinally. One and one-half inch (1.5″) of membrane ispeeled from the braid. A bare one inch (1″) section of braid is insertedinto the angle slot and the rest of the braid is bent around the wheelsuch that the longitudinal slot is facing toward the wheel surface. Theangled slot anchors one end and the loose end is placed in the floatingclamp arid tightened. Any slack is removed by the sample tensioningscrew. The loose end of the peeled section of membrane is placed in theupper clamp of the Tester. Four inches of membrane are pulled off thebraid at a rate of 100 mm/mm. The German wheel rotates freely to keepthe angle of peel constant. The material tester outputs a graph showingthe amount of force required to peel the membrane off the substrate. Theresults of the samples are averaged together and plotted on a graph. Theaverage maximum force of approximately the two inch section is recorded.Tensile Strength of Each Sample is conducted as follows:

The wet samples of membrane obtained from the Peel Test are placed inthe clamps of the Tester. The clamps are placed one inch apart. Themembrane sample is pulled apart at a rate of 100 mm/mm. The averagemaximum force for the samples is recorded along with the standarddeviation.

Cylindricity of the braid is determined by visual examination under amicroscope.

The asymmetric film comprises a very thin “skin” overlying a more porousstructure in which the pores are in open communication with one another.Such a membrane may be used for filtering either aqueous or non-aqueoussolvents. For filtration of a solvent such as a primary or secondaryalcohol, a ketone or a hydrocarbon, the polymer film is deposited from asolution of a solvent-resistant polymer such as polyacrylonitrile (PAN)or polyetheretber ketone (PEEK).

Referring to FIG. 2 there is shown a cross-sectional view of the coatingnozzle indicated generally by reference numeral 10, which, in additionto limiting the amount of dope (polymer in solution) passing through thenozzle, meters the correct amount of dope over the surface, anddistributes the metered amount uniformly over the surface of the braid(not shown) as it is drawn longitudinally axially through the nozzle.The nozzle 10 comprises an inner barrel 12 having an internal bore 13through which the braid is advanced into an axial bore 14 of a nipple 15which is threadedly secured in the end 16 of the inner barrel 12. Thebore 14 provides a rounding orifice to help the braid to acquire acircular cross-section before it is coated with dope. The roundingorifice 14 has a diameter in the range from about 1% to 10% less thanthe nominal diameter of the braid. The barrel 12 with the nipple 15 isinserted in an outer barrel member 20 having a cylindrical base 25. Theouter barrel 20 is provided with a stepped inner axial chamber with alarger bore 22 and a smaller bore 23 provided with threads (not shown)near the end of the bore 23. A top-hat bushing 27 having a stepped axialbore 27′ is threaded into the smaller bore 23 until it compresses anO-ring 27″ in a groove between the end of the barrel 20 and the lowerportion of the bushing. A sizing die 28 having a sizing orifice 24 ispress-fitted in the stepped axial bore 27′. The sizing orifice ensuresthe circularity of the cross-section of finished hollow membrane, uponleaving the rounding orifice. As the dope-coated braid is advancedthrough the sizing orifice, it dresses the outside diameter of thepolymer-coated surface to provide the dope with a desired wallthickness, which upon being coagulated, yields a thin film membranewhich is no more than 0.1 mm thick.

The base 25 is provided with a lower port 21 and an upper port 26 eachin open communication with the stepped bores 22 and 23, so that dopeintroduced into the port 21 can flow into the reservoir formed aroundthe inner barrel 12, by the stepped bores 22 and 23, and travellongitudinally axially in the direction in which the braid is drawnthrough the larger bore 22, and the smaller bore 23 displacing air asthe reservoir fills. When the dope having filled the reservoir flows outof the top port 26, it is plugged. The base 25 is removably secured withthrough-bolts (not shown) through the base 25 to a radially extendingmounting flange 29 having a longitudinal body portion 29′. The bodyportion 29′ is provided with an internally threaded axial bore so thatthe body portion 29′ can be secured coaxially in position, aligning therounding orifice 14 and the sizing orifice 24. By increasing ordecreasing the number of turns of the body portion 29′ the distancebetween the mouth of the orifice 14 and the orifice 24 can be varied.This distance is adjusted, depending upon the rate at which the braid ispulled through, the viscosity of the dope, and the thickness of the filmof dope to be coated on the braid before it is immersed in thecoagulant. In all cases, the distance is adjusted by trial and error, toprovide a film of dope on the circumferential outer surface of the braidonly sufficient to coat the braid superficially, and not enough to embedthe braid in the film.

To draw the braid through the orifice 24, a longitudinal tension ismaintained on the braid of at least I Newton but not enough to distortthe voids in the braid so badly that they cannot return to anequilibrium state as they are being coated with dope. Because the braidis not embedded in the viscous polymer solution, only the outer surfaceof the braid is contacted with the dope so as to provide the braid witha dope- and polymer-coated outer surface.

It will now be evident that the coating nozzle 10 is a special-purposenozzle specifically designed to provide a predetermined distance betweenthe rounding orifice 14 and the sizing orifice 24 while a dope coatedbraid no larger than about 2.5 mm (nominal O.D.) is advanced throughboth orifices sequentially. The amount of dope metered into the coatingnozzle and the rate at which the braid is advanced through the roundingorifice are determined by trial and error such as one skilled in thisart is accustomed to engage in under comparable circumstances.

After the dope-coated braid leaves the sizing orifice, it is led into acoagulating bath, typically under and over a series of rolls, so thatthe liquid coagulant held in the bath contacts the entirecircumferential surface of the coated braid. Because the polymer isinsoluble in the coagulant it does not penetrate the thin film formedand enter the lumen. Upon contacting the coagulant, the dope coagulates,yielding the desired thin film membrane. The bore of the fiber containsair at atmospheric pressure.

Referring to FIG. 3 there is shown in a diametrical cross-sectionalview, much enlarged, a tubular braid indicated generally by referencenumeral 30 comprising a braid of woven yarn 31 having a “lumen” (innerbore) 32. A thin film membrane, indicated generally by reference numeral33, is self-adherently secured to the circumferential outer surface 34which is rough and uneven because it is formed by the interwoven yarnwhich, in the range of thickness used and the number of picks in whichit is woven, does not result in an even surface. The essentialcharacteristic of the thin film membrane 33 is that it is supportedsuperficially, on the circumferential surface of the tubular braidwithout the braid becoming embedded in the thin film. Thischaracteristic is evident in a photomicrograph which clearly illustratesthat the circumferential inner surface of the tubular braid's bore 32 isessentially free of polymer.

Referring to FIG. 4 there is schematically illustrated, more greatlyenlarged than in FIG. 3, the asymmetric thin film membrane 33, whichwhen formed by being coagulated, is itself striated into an overlyingultrathin barrier layer or “skin” 35 and three distinctly identifiablelayers of pores, an outer layer 36, an inner layer 38 and anintermediate transport layer 37 between outer layer 36 and inner layer38, as schematically illustrated in greater detail in FIG. 4. The skinis a very thin dense layer of polymer formed as the dope contacts thecoagulant. By reason of the manner in which the skin and each layer isformed from the same polymer, the layers have, in a radially inwarddirection from under the skin to the braided yarn 39 which defines thebore 32, progressively larger pores. As shown in FIG. 4, each “end” 39or yarn consists of a multiplicity of filaments 39′, and thecircumferential surface of the interwoven strands of yarn does notprovide a smoothly cylindrical surface. The skin is generally thinnerand the pores for a MF membrane are larger than those of a UF membranemade from the same polymer. The measured skin thickness (by electronmicroscopy) for particular films made for the braided membrane is givenbelow to appreciate its thickness in relation to the pores of thelayers. The approximate ranges of sizes of the pores for preferred MFand UF membranes are given below: TABLE 2 MF, μM UF, μM Skin 35,thickness 0.1-1.5 1-4  Outer layer 36, avg pore diam 0.5-1.0 0.5-2  Intermediate transport layer 37 2-6 5-10 (average pore diameter) Innerlayer 38, avg pore diam 10-40 10-150

In membranes, in general, the thickness of the skin is small relative tothe thickness of the layers. The skin is thicker in a UP membrane thanin a MF membrane, and it would be even thicker in a RO membrane (notmeasured). Though FIG. 4 is not to scale, by reason of the manner inwhich the membrane is formed, the thickness of the outer layer isgenerally smaller than that of the transport layer, which in turn, isnot as thick as the inner layer. The approximate thickness of each layerin a MF and UP braided membrane are given in the following Table 3.TABLE 3 Thickness, average MF, μM UF, μM Skin 35 0.1-1.5  1-4 Outerlayer 36 5-10 20-40 Intermediate transport layer 37 30-50  40-80 Innerlayer 38 100-1000  100-1000

The following examples illustrate the invention, but should not beconstrued as limiting the invention which is defined in the appendedclaims.

EXAMPLE 1 Coating Braids with Different Properties with the Same Dope

In the following examples two tubular braids A and B, made from yarns ofnylon 6/6 fibers, and upon initial examination having properties whichare essentially the same except for the denier of the filament, are eachcoated with a dope, of poly(vinylidenefluoride) (PVDF) inN-methyl-2-pyrrolidone (NMP), containing a polyhydroxy alcoholhydrophilic additive and having a viscosity of 38,000 cps. The rate offlow of solution to the nozzle is adjusted so that the solution isflowed upon and around the periphery of the braid over a coatingdistance of 3 mm (0.125 inch). The braid, coated with the solution isthen pulled through a sizing die having a diameter of 2.5 mm, then ledinto a coagulation tank where the polymer solution is coagulated inwater to afford a semi permeable membrane about 0.06 mm thick, supportedon the tubular braid which assumes an essentially circularcross-section. It is then pulled through a glycerin bath, dried andtaken up onto the reel of a winder. The coating conditions for eachbraid are the same, namely:

-   -   Bath Temperature 46° C. (115° F.)    -   WUS 12.19 meters/mm (40 ft/mm)

The braids differed as follows: Braid A Braid B Yarn Denier 315 420Filaments 68 68 Denier/filament 4.6 6.2 Ends 1 1 Picks 44 44Cylindricity 0.9 0.9 Mean outside diam. 1.88 mm 2.01 mm Mean insidediam. 0.86 mm 1.06 mm Shrinkage 3.4% 3.4% Breaking strength, lb-f 5.937.68 Button (the inside diameter of a 2.15 mm 2.53 mm finishing diethrough which the coated braid is passed)

Upon being tested for filtration, coated Braid B provided a permeabilitytwice that of coated Braid A. Upon examination of the coated braids, itis found that Braid A, made with lower denier yarn, gave a looser” braidwhich allowed the dope to penetrate to the inner wall of the braid,embedding it, and leaving little on the outer surface, as is evidentfrom the following: Braid A Braid B Coated mean outer diam.  1.89 mm 2.15 mm Thickness of coating 0.005 mm 0.070 mm Mean wall thickness0.520 mm 0.475 mm Flux @ 15 psi 171.9 usgfd   383 usgfd

A photograph of a cross-section of the braided MF membrane, made with anelectron microscope, shows the film membrane overlying the braid to beabout 0.05 mm thick and the braid is not embedded in the film. Thethickness of the skin 35 and each individual layer 36-38 will dependupon the conditions under which the film is made. Measurements made in avertical plane through the circumference, across the wall of the film,provides the following data on pore sizes: Section Mm Skin thickness 0.8Outer layer 36 0.781 Intermediate layer 37 3 (average pore size) Innerlayer 38 14-32

The braided membrane was used to form a MF filtration module having aconstruction described in U.S. Pat. No. 5,783,083 to Mahendran et al.The water permeability measured under 67 kPa (5 psi suction pressure)and 22° C. is found to be 170 LMH (100 USgfd).

EXAMPLE 2 Comparison of Braids Made with Polyester and Glass Fiber Yarnsand an “ADC’ Membrane

A dope, code ADC, is made up similar to the PVDF-in-NMP solution used inExample 1 hereinabove, with 16 parts PVDF; 81 parts NMP; 2 parts HPVA;and 1 part LiCl; having a viscosity of 56,000 cps, and is fed to anozzle through which tubular braids of glass fiber and polyester areadvanced to prepare fibers which are substantially identical except forthe material of the yarn from which the braids are made. As before, theof flow of dope adjusted so that the solution is flowed upon and aroundthe periphery of each braid over a coating distance of 3 mm (0.125inch), pulled through the same sizing die, coagulated in water to afforda thin semipermeable membrane 0.05 mm thick, supported on the braid,then pulled through a glycerin bath. Each braided MF membrane has anO.D. of about 1.88-1.92 mm, and cylindricity of about 0.9, the I.D. ofeach being about 0.9 mm. Each coated braid is taken up onto the reel ofa winder and used to make skeins.

The skeins, each having an area of 1302 ft are placed in MF service in areservoir of water contaminated with leachate from a land-fill site. TheCOD of the leachate is in the range from 1000 to 1500 mg/L. Air in anamount in the range from 400 to 450 m³/hr is provided at the base ofeach skein. After six months service under usual operating conditionsand identical back-flushing procedures, it was found that every skeinmade with glass fiber braid bad suffered from 2 to 20 broken fibers.

When skeins made with fibers of polyester braid are placed in the sameservice as the fibers of glass fiber braid above, under identicaloperating conditions and the same back-flushing procedures, it was foundthat after six months service, not a single fiber of polyester braid wasbroken.

EXAMPLE 3

A dope is made up similar to the PVDF-in-NMP solution used in Example 1hereinabove, except that it is made up with the following components inthe relative amounts (parts by weight) set forth: N-methyl-2-pyrrolidone(NMP) 82; polyvinylidene fluoride (PVDF) 15; calcined a-aluminaparticles (“α-Al”) 2; 50% hydrolyzed polyvinyl acetate (HPVA) 1; for atotal of 100 parts.

70 g of calcined a-Al particles having an average primary particle sizeof about 0.4 μm are weighted in a flask to which 2787 g of NMP is addedand thoroughly mixed in a Sonicator® for at least 1.5 hr, to ensure thatagglomerates of primary particles are broken up so as to form asuspension in which individual primary particles are maintained inspaced apart relationship with each other in the NMP. The suspension ismilky white, the white color being contributed by the white calcinedα-Al. To this suspension is slowly added 525 g of PVDF having a numberaverage mol wt of about 30,000 Daltons while stirring at high speeduntil addition of the PVDF is complete. During the addition of the PVDFthe milky white color of the suspension changes first to pink, then toyellowish brown, at the end to grey/brown. Since PVDF dissolved in NMPproduces no color change, and the milky white color of the suspension isattributable to the α-Al particles, the changes in color provideevidence of a reaction between the calcined α-Al or a base present inthe calcined alumina.

When the grey/brown color of the NMP/PVDF/α-Al complex in suspension isstable and does not change upon standing for a sustained period in therange from 4 hr to 24 hr, 118 g of a 30% solution of 50% HPVA containing1.6-1.7% sulfuric acid in NMP is added to form a dope which is stirredovernight. The dope is then degassed either by letting it stand for 24hr, or by centrifuging it. The viscosity of the degassed dope is about14,500 centipoise (cp).

The dope formed is fed to a nozzle through which Braid B used in Example1 above is advanced at about 12.2 meters/mm (40 ft/mm), and coated at apressure of 274 kPa (25 psig) over a coating distance of 3 mm (0.125inch). The coated braid is sized in a sizing die having a diameter of2.55 mm, then led into a coagulation tank where the polymer solution iscoagulated in water to afford a semipermeable membrane about 0.13 mmthick, supported on the tubular braid which has a cylindricity of about0.9. This coated braid was then quenched by immersion in sequentialfirst and second coagulation baths of water, each at 47° C. (116° F.),and finally through a glycerin bath before it is taken up onto the reelof a winder. In tests, it is found that the braided MF membrane providesexcellent results.

After a section of the braided membrane was washed overnight in coldwater, its water permeability is determined by measuring its flux whichis found to be 6 LMH/kPa or, permeability of 25 GFD/psi measured at 5psi. After another section of the braided membrane, it is treated withan aqueous solution containing 2000 ppm of sodium hypochiorite (NaOCl).Water permeability of the NaOCl-treated membrane was found to be 12LMH/kPa measured at 35 kPa (50 GFD/psi measured at 5 psi). In each case,the pore size measurements and molecular weight cut-off measurementsprovide evidence that the pores in the film are suitable formicrofiltration.

1. A separation membrane comprising, (a) a tubular braid support for ahollow fiber separation membrane made from 16 to 60 separate yarns, eachyarn being between 150 and 400 denier, the tubular braid support havingan outside diameter between 1.5 and 2.5 mm and a wall thickness greaterthan 0.2 mm and less than 1.0 mm, the tubular braid support having atleast 30 picks per inch; and, (b) a porous substance attached to thesupport, the porous substance covering the outer circumferential surfaceof the support, the porous substance being between 0.05 and 0.3 mm thickbeyond the outer surface of the support and having pores suitable foruse as a separation membrane.
 2. The membrane of claim 1 wherein theyarns are shrunken to a stable length before the porous substance isattached to the support and the support with shrunken yarns has anextension at break of at least 10%.
 3. The membrane of claim 1 whereinthe support has a pre-shrunk length that is at least 1% less than anun-shrunk length of the support.
 4. The membrane of claim 3 wherein thesupport has a pre-shrunk length that is between 1% and 20% less than anun-shrunk length of the support.
 5. The membrane of claim 4 wherein thesupport has a pre-shrunk length that is between 1% and 8% less than anun-shrunk length of the support.
 6. The membrane of claim 1 wherein thesupport is flexible and macroporous.
 7. The membrane of claim 2 whereinthe support has an extension at break of at least 20%.
 8. The membraneof claim 1 wherein the air permeability of the support without theporous substance attached is at least 1 cc/sec/cm² at 1.378 kPa.
 9. Themembrane of claim 8 wherein the air permeability of the support withoutthe porous substance attached is less than about 10 cc/sec/cm² at 1.378kPa.
 10. The membrane of claim 1 wherein the support is not embedded inthe porous substance.
 11. The membrane of claim 1 wherein the support iswoven with from 1 to 3 multifilament ends.
 12. The membrane of claim 11wherein the support comprises a multifilament made with from 40 to 100filaments, each filament being from 3 to 6 denier.
 13. The membrane ofclaim 1 wherein the yarns are woven in a pattern selected from Diamond,Regular or Hercules.
 14. The membrane of claim 1 wherein the supportcomprises a tubular braid woven with yarns having a plurality ofmultifilament ends, and the ends are non-plied in each yarn but lielinearly adjacent each other until taken up to form the braid.
 15. Themembrane of claim 1 wherein the porous substance has pores of a sizesuitable for use as a microfiltration or ultrafiltration membrane. 16.The membrane of claim 1 wherein the support has a moisture regain of0.2% to 7% by weight.
 17. A separation membrane comprising, (a) asupport for a hollow fiber separation membrane made from yarns braidedinto a tube, each yarn being between 200 and 400 denier, with from 16 to60 carriers; and, (b) a porous substance attached to and covering theouter circumferential surface of the support and having pores suitablefor use as a separation membrane, wherein, the support has at least 36crosses per inch, an outside diameter of between 1.5 mm and 2.5 mm and awall thickness of more than 0.15 mm and less than 0.5 mm.
 18. Themembrane of claim 17 wherein the support comprises at least 16 separateyarns and is woven with from 1 to 3 multifilament ends.
 19. The membraneof claim 18 wherein the support comprises a multifilament made with from40 to 100 filaments, each filament being from 3 to 6 denier.
 20. Themembrane of claim 17 wherein the yarns are woven in a pattern selectedfrom Diamond, Regular or Hercules.
 21. The membrane of claim 17 whereinthe support comprises a tubular braid woven with yarns having from 1 to3 non-plied multifilament ends.
 22. The membrane of claim 17 wherein theair permeability of the support without the porous substance attached isless than about 10 cc/sec/cm² at 1.378 kPa.
 23. The membrane of claim 17wherein the yarns are pre-shrunken to a stable length.
 24. The membraneof claim 17 wherein the support has an extension at break of at least10%.
 25. The membrane of claim 17 wherein the support has a pre-shrunklength that is at least 1% less than an un-shrunk length of the support.26. The membrane of claim 25 wherein the support has a pre-shrunk lengththat is between 1% and 20% less than an un-shrunk length of the support.27. The membrane of claim 26 wherein the support has a pre-shrunk lengththat is between 1% and 8% less than an un-shrunk length of the support.28. The membrane of claim 17 wherein the support is flexible andmacroporous.
 29. The membrane of claim 17 wherein the support has anextension at break of at least 20%.
 30. The membrane of claim 17 whereinthe porous substance is at least 0.05 mm thick from the outer surface ofthe support to the outside of the porous substance.
 31. The membrane ofclaim 30 wherein the porous substance is between 0.05 mm and 3.0 mmthick beyond the outer surface of the support to the outside of theporous substance.
 32. The membrane of claim 17 wherein the poroussubstance is less than 2.0 mm thick beyond the outer surface of thesupport to the outside of the porous substance.
 33. The membrane ofclaim 17 wherein the support is not embedded in the porous substance.34. The membrane of claim 17 wherein the porous substance has pores of asize suitable for use as a microfiltration or ultrafiltration membrane.35. The membrane of claim 17 wherein the support has a moisture regainof 0.2% to 7% by weight.
 36. The membrane of claim 1 wherein each yarnis on its own carrier.
 37. The membrane of claim 17 wherein the supporthas a rough and uneven surface formed by the overlapping yarns.